The research outlined in this proposal concerns the application of spectroscopic methods to studies on the light-induced electron transfer reactions of photosynthetic membranes. Answers are sought to questions such as what role does the organization of neighboring electron donors and acceptors within the membrane-bound protein-lipid complexes of photosynthetic membranes play in the overall goal of rapid, efficient, charge separation and stabilization and how is distance of separation and availability of paths for orbital overlap between cofactors involved in controlling electron transfer direction and rates? Studies of Photosystem I of spinach are suggested in which electron spin polarization of the light-induced paramagnetic intermediates is used to characterize the identity of the acceptors, their mutual interactions, relative orientation, and electron transfer rates. Reaction center membrane fragments from bacteria are used to examine the effect of replacement of Fe(II) by Mn(II) in the quinone complex on the electron transfer kinetics between the quinones. Electron transfer kinetics are measured by electron paramagnetic resonance, electron spin echo and optical absorbance spectroscopies. Kinetics will be correlated with the magnitude of the electron spin exchange interactions between paramagnetic cofactors. Orientation will be measured in terms of the anisotropic magnetic properties of those cofactors.